I have an NSTableView, two columns one with a NSString, the other with a NSImage. I need an action that changes the NSImage column, of a selected row from its original image to a new one.
What do you mean?
You can react to the table view’s selection changing (as described in the answer to your question of a few days ago) by changing the image you return for - (id)tableView:(NSTableView *)aTableView objectValueForTableColumn:(NSTableColumn *)aTableColumn row:(int)rowIndex
You’d learn a lot more about Cocoa by actually writing this application yourself, instead of asking everyone here to write it for you.